In subterranean engineering, i.e. the construction of tunnels, galleries or shafts, it is frequently desirable to stabilize the excavated structure by anchoring wall or support members thereto and, for this purpose, drilling and bolting machines have been developed as, for example, will be apparent from the aforementioned applications and patents.
The procedure which can be carried out with the aid of such machines involves the drilling of a so-called borehole and the subsequent anchoring of a bolt therein. Frequently the anchoring of the bolt can be effected at least in part by introducing into the hole in which the bolt is inserted, a hardenable synthetic resin mass, especially a thermosetting synthetic resin.
For this procedure therefore, in the field of mining applicances, slides are known which are carried by support arms and along which a carriage operating as a drill and screwing arrangement is able to move, which makes it possible to drill a hole by means of a bit, then to introduce and tighten an anchoring bolt in the hole drilled previously.
During the operation of drilling the hole, the bit passes through a device referred to as a "bit guide", mounted at the front end of the slide. During the subsequent bolting operation, this bit guide must be retracted, in order to clear a sufficient space for the passage of the bolt with its customary bearing plate. To this end, retractable bit guides already exist. Such guides have a body through which the bit may pass and which can be moved by pivoting towards a retracted position, by a mechanism comprising a jack.
In retractable state-of-the-art bit guides of this type, the jack mechanism comprising a jack ensures virtually only a control of the opening and closing movement, but does not produce adequate hydraulic tightening in the closed position, for drilling, owing to the small lever arm, in part caused by the need to make the device as small as possible. The necessity of clearing a sufficient space for the passage of bolts provided with their plate, when the bit guide is retracted, makes it impossible to install in this space a jack which can ensure good locking. The result is that in earlier bit guides, the retraction function is the only function ensured and no locking is provided; this constitutes a serious drawback since the bit guides are not held sufficiently firmly in order to withstand the considerable forces transmitted by the bits during drilling.